In general, an electrode-type water level detector is used for such purposes as determining water levels or detecting leakage of water.
FIG. 18 is a schematic diagram illustrating a conventional electrode-type water level detector. FIG. 19 is a circuit diagram illustrating the conventional electrode-type water level detector.
As illustrated in FIG. 18, the conventional electrode-type water level detector includes a first water level measurement electrode 2 and a second water level measurement electrode 3, which are insulated from each other, and a relay unit 4. A to-be-measured object 1 is a conductor that conducts electricity, such as water. When both the first water level measurement electrode 2 and the second water level measurement electrode 3 come in contact with the to-be-measured object 1, current flows between the first water level measurement electrode 2 and the second water level measurement electrode 3 via the to-be-measured object 1. The flow of the current through the relay unit 4 activates the relay unit 4, and whether or not the to-be-measured object 1 exists is detected as a result. In this case, the to-be-measured object 1 serves as an electrical resistor which connects the first water level measurement electrode 2 and the second water level measurement electrode 3.
As illustrated in FIG. 19, the relay unit 4 includes a threshold adjustment unit 6, a relay coil 7, and a relay switch 8. The threshold adjustment unit 6, the relay coil 7, and the relay switch 8 do not come in contact with the to-be-measured object 1, and are therefore more unlikely to corrode than the first water level measurement electrode 2 and the second water level measurement electrode 3. Accordingly, it is assumed that a change in the resistance value is small, and that there is no loss. Current (I) that flows through the circuit is expressed by the following formula (1):I=V/(Z11+Z12+Z)  (1)
In the formula (1), Z11 is a first resistance value representing the resistance of the first water level measurement electrode 2, Z19 is a second resistance value representing the resistance of the second water level measurement electrode 3, and Z is a resistance value of the to-be-measured object 1, and V represents a voltage applied from a constant voltage source a.
When the current flowing through the relay coil 7 exceeds a relay operation threshold value, the relay switch 8 is activated. Then, it is determined that the first water level measurement electrode 2 and the second water level measurement electrode 3 are in contact with the to-be-measured object 1. In this case, the current or voltage flowing between the first water level measurement electrode 2 and the second water level measurement electrode 3 is measured by the threshold adjustment unit 6, and a threshold value of the resistance value of the to-be-measured object 1 is set as an invariant point. This enables the to-be-measured object 1 to be detected depending on the type or state of the to-be-measured object 1.
Among those that utilize such a detection method is a technique disclosed in Patent Document 1, for example. According to this technique, a water level measurement electrode is moved so that the level of water is continuously detected. When the level is not being measured, the water level measurement electrode is separated from the to-be-measured object, thereby preventing effects of the water level measurement electrode.
A technique disclosed in Patent Document 2 uses a conductive refractory to prevent current from going around into a furnace body. Moreover, the technique disclosed in Patent Document 2 corrects change of an electric resistance value of the conductive refractory by temperature to improve the sensitivity of detection.
As a similar detection method, there is a method of utilizing not voltage but capacitance in order to detect the water level. Among those that use this method is a technique disclosed in Patent Document 3, for example. This technique gets the frequency of AC voltage, which is used to measure capacitance, to change; a change in the dielectric constant of the to-be-measured object and insulating material is thereby calculated. Then, the measured change in the electrode capacitance is corrected with the dielectric constant of the insulating material and the dielectric constant of the medium, and the level is calculated. In this manner, a higher level of sensitivity is achieved.